Configure GRUB for dual boot

WOL DualBoot can choose the operating system for the next boot when GRUB manages the boot process, contains valid Linux and Windows entries and provides grub-reboot. The agent checks these requirements, but it does not install or modify GRUB for you.

These steps change the boot loader. Run them from Linux and keep a backup of the configuration. If the computer does not currently boot through GRUB, consult your distribution documentation first: installing or repairing the loader depends on whether the computer uses UEFI or legacy BIOS.

1. Check that GRUB is available

Run these commands from Debian or Ubuntu:

command -v grub-reboot
sudo test -r /boot/grub/grub.cfg && echo "grub.cfg available"

The first command should print a path, usually /usr/sbin/grub-reboot. If it does not, check that GRUB is installed correctly before continuing.

Back up the editable configuration:

sudo cp /etc/default/grub /etc/default/grub.woldualboot.bak

2. Check the Linux and Windows entries

Regenerate the menu and inspect the detected entries:

sudo update-grub
sudo grep -E "^[[:space:]]*menuentry " /boot/grub/grub.cfg

At least one Linux entry and one Windows entry, usually named Windows Boot Manager, must be present.

If Windows is missing, first check that both systems were installed in the same firmware mode—normally UEFI. On Debian/Ubuntu you can also install and run os-prober:

sudo apt update
sudo apt install os-prober
sudo os-prober

If os-prober detects Windows but update-grub does not add it, set this option in /etc/default/grub:

GRUB_DISABLE_OS_PROBER=false

Then run sudo update-grub again. Enable os-prober only on your own trusted computer because it inspects other connected file systems.

3. Enable one-time boot selection

Open /etc/default/grub as an administrator and make sure it contains:

GRUB_DEFAULT=saved

Apply the change:

sudo update-grub

Set Linux as the normal destination using the exact name of its entry. On Ubuntu it is usually Ubuntu:

sudo grub-set-default "Ubuntu"

This lets grub-reboot select Windows for the next boot only, while a later restart from Windows returns to the saved Linux default.

4. Test grub-reboot

Use the exact entry name shown in grub.cfg:

sudo grub-reboot "Windows Boot Manager"
sudo grub-editenv list

The output should contain a next_entry variable. When ready, restart the computer and check that it enters Windows. The choice is consumed once; the following restart should return to the configured Linux default.

To cancel the test before restarting:

sudo grub-editenv - unset next_entry

5. Check WOL DualBoot Agent

Open the agent tray app on Linux and press Refresh. The boot section should show the grub loader, Linux and Windows entries, the grub.cfg path and actions for booting Linux or Windows.

If an action is unavailable, diagnostics indicate whether grub-reboot, a menu entry or access to the configuration is missing.

Windows limitation

Windows cannot robustly select an internal GRUB entry. Leaving Linux as the default is therefore recommended: from Linux, WOL DualBoot schedules Windows for one boot; from Windows, a normal restart returns the computer to Linux.

For all available options, see the official GRUB simple configuration and grub-reboot reference.